Project Abstract The goal of this project is to characterize the role of sleep in the onset of perinatal depression (i.e., depression occurring during pregnancy and the first twelve months postpartum). Disrupted sleep is well established as a symptom of new and recurrent depression in non-pregnant samples, but remains understudied in the perinatal period. This period of significant hormonal, physiologic and psychosocial change contributes to poorer subjective (i.e., self-reported) sleep quality. However, the relationship between objective sleep characteristics and perinatal depression remains less studied, especially during pregnancy. Such alterations in habitual sleep characteristics have been posited to contribute to postpartum depression. Heightened daily stress appraisal may be one mechanism through which deficient sleep leads to prodromal depression in the perinatal period. To this end, the current project aims to characterize the bidirectional relationship between sleep characteristics and mood during both the pregnancy and postpartum periods through prospective objective sleep assessment paired with daily ecological momentary assessment (EMA) of stress, and proposes three specific aims: (1) To examine the relationship between prenatal and postpartum sleep patterns, using a within-person design that makes use of both objective and subjective measures. (2) To examine the relationship between sleep and mood during the prenatal and postpartum period, using innovative EMA methods to track daily stress appraisal alongside objective sleep assessment. (2.1) Establish the relationship between sleep patterns and stress appraisals. (2.2) Determine whether daily stress appraisals explain the relationship between sleep and depression. (3) Determine the extent to which prenatal sleep patterns predict longitudinal assessments of postpartum depression, independent of prenatal depression symptoms. To accomplish these research aims, we will recruit 50 women without a history of depression or sleep disorders from the Obstetrics and Gynecology Division at the George Washington University Hospital System. Participants will complete a ten-day, nine-night protocol, conducted during pregnancy (third trimester week 28) and postpartum (week 6). Validated measures of subjective sleep and perinatal depression will be administered during pregnancy and at two weeks, six weeks, and three months postpartum. The research team has developed a novel daily diary phone application involving EMA appraisals of stress, that is specific to the perinatal period. Together, these methodological innovations can inform models of how and when sleep impacts mood among pregnant and postpartum women. This knowledge is critical to advancing our understanding of PD onset. It also carries implications for how we assess stress and negative affect in pregnancy and postpartum, and can inform clinical practice and targeted intervention development.